We have designed ultra-thin graphene microwave transistors by using pre-patterned metal or graphene nanoribbon back-gates and hexagonal boron nitride as a dielectric substrate. Despite the inhomogeneities induced by the graphene transfer process, we show that it is possible to operate these types of devices across a broad range of microwave frequencies. For the graphene nanoribbon gates, we observe a deviation of the current gain from the usual 1/f trend that can be attributed to the large gate resistance of these systems as we demonstrate with our small-signal model. The scattering parameter analysis shows a very limited back-action from the channel onto the graphene nanoribbon gates. Our work thus proves that graphene microwave transistors could be driven by graphene nanoribbon gates.
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In this small-signal model, we neglect all inductances (including kinetic inductance) and series resistances in drain and source since they mainly scale up or down, and do not affect dramatically the value of Rg.
This is due to the wider channel and thinner h-BN thickness of the graphene gate devices as to compare to the metal gate devices (and therefore increases the gate coupling capacity).
